It is known that manganous-manganic oxide (Mn.sub.3 O.sub.4) can readily be prepared by either reduction of a higher oxide of manganese such as manganese (III) oxide, a manganese (IV) oxides and MnOOH, or by oxidation of a lower oxide or hydroxide of manganese such as manganese (II) oxide and manganese (II) hydroxide. Representative examples of such known processes are those described in U.S. Pat. Nos. 4,411,878 and 3,767,780. In U.S. Pat. No. 4,411,878 there is disclosed a process for the preparation of manganous-manganic oxide comprising the reductive roasting of a higher oxide of manganese in the presence of a reducing gas, specifically methane. In accordance with the teachings of this patent, a higher oxide of manganese such as MnO.sub.2, Mn.sub.2 O.sub.3 or MnOOH, and the like, is heated in a kiln in the presence of an excess current of methane gas. Operating temperatures are limited to levels between 250.degree. C. and 550.degree. C. to prevent further reduction of the Mn.sub.3 O.sub.4 product formed to manganese (II) oxide. The advantages of this process are asserted to be that it is simpler to carry out than alternate wet processes involving the precipitation of manganese (II) hydroxide and the subsequent oxidation of this hydroxide to the desired manganous-manganic oxide product. The process also is asserted to offer significant advantages over other dry reduction processes employing temperatures higher than 550.degree. C. For example, higher temperature dry reduction processes result in the production of manganese (II) oxide which then must be reoxidized to the desired manganous-manganic oxide product. Also, the resulting manganous-manganic oxide product has a lower surface area and activity due to sintering.
U.S. Pat. No. 3,767,780 discloses a wet process for the preparation of manganous-manganic oxide comprising oxidation of an aqueous suspension of manganese (II) hydroxide at elevated temperatures and at superatmospheric pressures. In general, the operating temperatures and pressures employed in this patented process range from 100.degree. C. to 140.degree. C. and from 40 psi to 70 psi, respectively. The advantages of this process over other wet oxidation processes are asserted to be that reaction times are shorter, the manganous-manganic oxide product is more readily filtered and the product is less contaminated. In the main, these advantages can be and are attributable by the patentees to the use of superatmospheric pressures during the oxidation reaction.